Optical images produced by an optical scanning device are susceptible to anomalies that affect the integrity of the optical image. Such anomalies include striping, which refers to optical image artefacts that can manifest, for example as lines on the optical image in the direction of scanning.
Typically, the direction of scanning is vertically down a target area, which may produce striping vertically down the optical image, and striping may therefore be termed vertical striping. For another example it also possible, however, that striping may occur horizontally if the direction of scanning of the optical scanning device is horizontally across the target area.
When an optical scanning device produces an optical image a target area is scanned by for example a laser over a plurality of scan lines. Light is reflected from the target to allow the optical scanning device to produce an optical image. The target may be, for example, the retina of a human or animal subject, an existing image such as a photograph or any other object or surface of which an optical image is desired to be created.
In certain optical scanning devices, a laser is passed vertically over a target area to produce a first scan line. The laser is then moved horizontally in one direction and passed vertically over the target area again to produce a second scan line adjacent the first scan line. This process is repeated until all of the target area has been scanned. A polygonal mirror may be used in optical scanning devices of this type, each facet of the mirror being used to move the laser beam over a single scan line.
If one or more facets of a polygonal mirror are defective then a striping anomaly may occur. The striping anomaly may exhibit anomalous attenuated or accentuated brightness of one line of an optical image relative to adjacent lines of the optical image because a defective facet may disperse light from the laser beam such that a portion of the light is not reflected from the polygonal mirror onto the target area and therefore is not reflected from the target area to produce the correct/desired intensity of optical image data.
Striping can result in the polygonal mirror or optical scanning device being rejected. This results in a significantly reduced yield with a high cost to manufacturers of polygonal mirrors and optical scanning devices.
Current known solutions for determining the level of striping in an optical image include human observation in which an operative manually determines the level of striping by eye. If the level of striping is considered to be too high then the polygonal mirror or optical scanning device may be rejected.